(1) Field of the Invention
This invention relates to a device for applying pressure to stacked sheets, cards, plates or like materials which are to be individually drawn off from a stack. Such a device is a constituent of a sheet feeding apparatus which is used particularly in copying machines and reenlargement apparatus. Also, such sheet feeding apparatus is used particularly in electronic processors in which copying material in the form of sheets, cards, or finished data carriers, such as punched cards, are individually fed from the stack to the machine for further processing. Furthermore, in the field of microfilm application as well as in the reading or duplicating of punched cards for electronic computers, vertical stack arrangements are used often today for handling material to be fed to the machine.
(2) Description of the Prior Art
In a known feeding device, the sheet or card stack is moved vertically upwardly by means of a slide actuated by a motor via a transmission system. Use of a motor and transmission, makes such a device expensive in its construction and also susceptible to trouble because of the electromotor.
Another known device for issuing individually succeeding cards from one end of a card stack has a load member upon which acts a counterholding device in order to permit only one movement of the load member to that end of the stack from which the cards are issued. The counterholding device substantially prevents movement of the load member in the opposite direction. The counterholding device is provided with two leaf springs which are carried by the load member and extend obliquely to the lateral surfaces of the magazine for the reception of the stack. It is again a vertical stack of which the lowermost card to be drawn off by an issuing mechanism is constantly loaded with a pressure resulting from the sum of the stacked cards arranged thereupon and the weight of the load member. It can be seen that the contact pressure on the card to be issued is not constant but depends on the height of the card stack.
An essential criterion for perfect operation of an issuing mechanism of a sheet feeding device is that a constant contact pressure is exerted upon the first as well as upon the last card to be drawn off from the stack. This is particularly necessary with high-speed machines in which drawing off of the sheets or cards is performed continuously by means of friction rolls. In the case of fluctuations of the contact pressure, double introductions easily occur. That means that two or more cards, sheets, microfilms or plates are drawn off simultaneously. Such simultaneous multiple feeding actuates control devices of the machine which, in turn, actuate a trouble indicator, bringing the machine to a standstill. In the case of machines without co-ordinated devices for indicating trouble and stopping the machine, simultaneous withdrawal of two or more sheets, microfilms or the like generally leads to undersirable incorrect exposures. Another disadvantage is that the drawn-off material, in the case of double introduction, is generally more or less damaged, which requires manual removal thereof from the travel path to ensure perfect operation of the machine. This generally also requires stopping of the machine.
In the case of vertical stacking, a constant contact pressure is generally achieved by loading the stack with a corresponding weight. Despite this measure, the contact pressure is different for the individual cards or sheets of the stack since, between the first and the last fed card, the contact pressure is reduced by the deviation of the stack weight resulting from the differing height of the stack.
In the case of horizontal stacking, the contact pressure is achieved by springs resting with one end upon the rear side of a slide member and with the other end upon a back wall of the device. The springs must have a spring characteristic as flat as possible but, despite various different arrangements and combinations for achieving a flat spring characteristic, there is still a considerable pressure difference of the order of a few kilograms as the stack becomes depleted. Another disadvantage is that the flat spring characteristic requires a considerable spring length, so that much space is required for horizontal stacking of the material to be stacked. Tests have been made to compensate for the great difference of the contact pressure between the first and the last stack feedings by a very exact adjustment of the spring tension, but it has been shown that, despite the high expenditure for exact adjustment of the spring tension, double introductions and the difficulties attendant thereto can be successfully prevented only in the medium adjustment range of the springs but not for the marginal ranges of the spring characteristic.